Information on the movement of cetaceans in relation to survey vessels is important for determining if abundance and density estimates from ship-based line transect sighting surveys are biased. A ship-based transect survey was conducted for harbor porpoise, Phocoena phocoena, in the western Bay of Fundy during the summer of 1988. Eight hundred and twenty four observations were made on swimming directions of porpoises at the time of sighting. Results suggest that harbor porpoises exhibit avoidance behavior. Animals were observed to be swimming in directions away from the trackline significantly more frequently than towards it. and this behavior was more frequent for animals sighted within 400m of the vessel than for animals seen at greater distances.
These results are shown not to be due to differences in glare on opposite sides of the vessel, but could possibly be due in part to the high swimming speed of the porpoises relative to the sighting platform. The likelihood of this possibility should be evaluated using simulation techniques. The potential biases that avoidance behavior can induce in line transect estimates of abundance are severe and thus data on swimming direction should routinely be collected during shipboard line transect surveys, and tests for avoidance and attraction behaviors using these data should be developed.